Despite its prevalence in the environment, no known form of life uses aluminum saltsmetabolically, but aluminum is well tolerated by plants and animals.[8] Because of their abundance, the potential for a biological role is of continuing interest and studies continue.

"Bauxite tailings" storage facility in Stade, Germany. The aluminum industry generates about 70 million tons of this waste annually.

Physical

Aluminum is a relatively soft, durable, lightweight, ductile, and malleablemetal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is nonmagnetic and does not easily ignite. A fresh film of aluminum serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared radiation. The yield strength of pure aluminium is 7–11 MPa, while aluminum alloys have yield strengths ranging from 200 MPa to 600 MPa.[9] Aluminum has about one-third the density and stiffness of steel. It is easily machined, cast, drawn and extruded.

Corrosion resistance can be excellent because a thin surface layer of aluminum oxide forms when the bare metal is exposed to air, effectively preventing further oxidation,[12] in a process termed passivation. The strongest aluminum alloys are less corrosion resistant due to galvanic reactions with alloyed copper.[9] This corrosion resistance is greatly reduced by aqueous salts, particularly in the presence of dissimilar metals.

In highly acidic solutions, aluminum reacts with water to form hydrogen, and in highly alkaline ones to form aluminates— protective passivation under these conditions is negligible. Primarily because it is corroded by dissolved chlorides, such as common sodium chloride, household plumbing is never made from aluminum.[13]

However, because of its general resistance to corrosion, aluminum is one of the few metals that retains silvery reflectance in finely powdered form, making it an important component of silver-colored paints. Aluminum mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3,000–10,000 nm (far IR) regions; in the 400–700 nm visible range it is slightly outperformed by tin and silver and in the 700–3000 nm (near IR) by silver, gold, and copper.[14]

This conversion is of interest for the production of hydrogen. However, commercial application of this fact has challenges in circumventing the passivating oxide layer, which inhibits the reaction, and in storing the energy required to regenerate the aluminum metal.[15]

Aluminum has many known isotopes, with mass numbers range from 21 to 42; however, only 27Al (stable) and 26Al (radioactive, t1⁄2 = 7.2×105 years) occur naturally. 27Al has a natural abundance above 99.9%. 26Al is produced from argon in the atmosphere by spallation caused by cosmic-rayprotons. Aluminum isotopes are useful in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of 26Al to 10Be has been used to study transport, deposition, sediment storage, burial times, and erosion on 105 to 106 year time scales.[16]Cosmogenic26Al was first applied in studies of the Moon and meteorites. Meteoroid fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial 26Al production. After falling to Earth, atmospheric shielding drastically reduces 26Al production, and its decay can then be used to determine the meteorite's terrestrial age. Meteorite research has also shown that 26Al was relatively abundant at the time of formation of our planetary system. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.[17]

Stable aluminum is created when hydrogen fuses with magnesium, either in large stars or in supernovae.[18] It is estimated to be the 14th most common element in the Universe, by mass-fraction.[19] However, among the elements that have odd atomic numbers, aluminum is the third most abundant by mass fraction, after hydrogen and nitrogen.[19]

In the Earth's crust, aluminum is the most abundant (8.3% by mass) metallic element and the third most abundant of all elements (after oxygen and silicon).[20] The Earth's crust has a greater abundance of aluminum than the rest of the planet, primarily in aluminum silicates. In the Earth’s mantle, which is only 2% aluminum by mass, these aluminum silicate minerals are largely replaced by silica and magnesium oxides. Overall, the Earth is about 1.4% aluminum by mass (eighth in abundance by mass). Aluminum occurs in greater proportion in the Earth than in the Solar system and Universe because the more common elements (hydrogen, helium, neon, nitrogen, carbon as hydrocarbon) are volatile at Earth's proximity to the Sun and large quantities of those were lost.

Because of its strong affinity for oxygen, aluminum is almost never found in the elemental state; instead it is found in oxides or silicates. Feldspars, the most common group of minerals in the Earth's crust, are aluminosilicates. Native aluminum metal can only be found as a minor phase in low oxygen fugacity environments, such as the interiors of certain volcanoes.[21] Native aluminum has been reported in cold seeps in the northeastern continental slope of the South China Sea. Chen et al. (2011)[22] propose the theory that these deposits resulted from bacterialreduction of tetrahydroxoaluminate Al(OH)4−.[22]

Although aluminum is a common and widespread element, not all aluminum minerals are economically viable sources of the metal. Almost all metallic aluminum is produced from the ore bauxite (AlOx(OH)3–2x). Bauxite occurs as a weathering product of low iron and silica bedrock in tropical climatic conditions.[23] Bauxite is mined from large deposits in Australia, Brazil, Guinea, and Jamaica; it is also mined from lesser deposits in China, India, Indonesia, Russia, and Suriname.

The intermediate, sodium aluminate, with the simplified formula NaAlO2, is soluble in strongly alkaline water, and the other components of the ore are not. Depending on the quality of the bauxite ore, twice as much waste ("Bauxite tailings") as alumina is generated.

The conversion of alumina to aluminum metal is achieved by the Hall–Héroult process. In this energy-intensive process, a solution of alumina in a molten (950 and 980 °C (1,740 and 1,800 °F)) mixture of cryolite (Na3AlF6) with calcium fluoride is electrolyzed to produce metallic aluminium:

Al3+ + 3 e− → Al

The liquid aluminum metal sinks to the bottom of the solution and is tapped off, and usually cast into large blocks called aluminum billets for further processing. Oxygen is produced at the anode:

2 O2− + C → CO2 + 4 e−

The carbon anode is consumed by reaction with oxygen to form carbon dioxide gas, with a small quantity of fluoride gases. In modern smelters, the gas is filtered through alumina to remove fluorine compounds and return aluminum fluoride to the electrolytic cells. The anode this reduction cell must be replaced regularly, since it is consumed in the process. The cathode is also eroded, mainly by electrochemical processes and liquid metal movement induced by intense electrolytic currents. After five to ten years, depending on the current used in the electrolysis, a cell must be rebuilt because of cathode wear.

World production trend of aluminum

Aluminum electrolysis with the Hall–Héroult process consumes a lot of energy. The worldwide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminum produced (52 to 56 MJ/kg). Some smelters achieve approximately 12.8 kW·h/kg (46.1 MJ/kg). (Compare this to the heat of reaction, 31 MJ/kg, and the Gibbs free energy of reaction, 29 MJ/kg.) Minimizing line currents for older technologies are typically 100 to 200 kiloamperes; state-of-the-art smelters operate at about 350 kA. Trials have been reported with 500 kA cells.[citation needed]

The Hall–Heroult process produces aluminum with a purity of above 99%. Further purification can be done by the Hoopes process. This process involves the electrolysis of molten aluminum with a sodium, barium and aluminum fluoride electrolyte. The resulting aluminum has a purity of 99.99%.[8][24]

Electric power represents about 20% to 40% of the cost of producing aluminum, depending on the location of the smelter. Aluminum production consumes roughly 5% of electricity generated in the U.S.[25] Aluminum producers tend to locate smelters in places where electric power is both plentiful and inexpensive—such as the United Arab Emirates with its large natural gas supplies,[26] and Iceland[27] and Norway[28] with energy generated from renewable sources. The world's largest smelters of alumina are located in the People's Republic of China, Russia and the provinces of Quebec and British Columbia in Canada.[25][29][30]

Aluminum spot price 1987–2012

In 2005, the People's Republic of China was the top producer of aluminium with almost a one-fifth world share, followed by Russia, Canada, and the US, reports the British Geological Survey.

Over the last 50 years, Australia has become the world's top producer of bauxite ore and a major producer and exporter of alumina (before being overtaken by China in 2007).[29][31] Australia produced 77 million tonnes of bauxite in 2013.[32] The Australian deposits have some refining problems, some being high in silica, but have the advantage of being shallow and relatively easy to mine.[33]

The high energy consumption of Hall–Héroult process motivated the development of the electrolytic process based on aluminum chloride. The pilot plant with 6500 tons/year output was started in 1976 by Alcoa. The plant offered two advantages: (i) energy requirements were 40% less than plants using the Hall–Héroult process, and (ii) the more accessible kaolinite (instead of bauxite and cryolite) was used for feedstock. Nonetheless, the pilot plant was shut down. The reasons for failure were the cost of aluminum chloride, general technology maturity problems, and leakage of the trace amounts of extremely toxic polychlorinated biphenyl compounds.[34][35]

Aluminum chloride process can also be used for the co-production of titanium, depending on titanium contents in kaolinite.

The non-electrolytic aluminum carbothermic process of aluminum production would theoretically be cheaper and consume less energy. However, it has been in the experimental phase for decades because the high operating temperature creates difficulties in material technology that have not yet been solved.[36][37]

Aluminum is theoretically 100% recyclable without any loss of its natural qualities. According to the International Resource Panel's Metal Stocks in Society report, the global per capita stock of aluminum in use in society (i.e. in cars, buildings, electronics etc.) is 80 kg (180 lb). Much of this is in more-developed countries (350–500 kg (770–1,100 lb) per capita) rather than less-developed countries (35 kg (77 lb) per capita). Knowing the per capita stocks and their approximate lifespans is important for planning recycling.

Recovery of the metal through recycling has become an important task of the aluminum industry. Recycling was a low-profile activity until the late 1960s, when the growing use of aluminum beverage cans brought it to public awareness.

Recycling involves melting the scrap, a process that requires only 5% of the energy used to produce aluminum from ore, though a significant part (up to 15% of the input material) is lost as dross (ash-like oxide).[38] An aluminum stack melter produces significantly less dross, with values reported below 1%.[39] The dross can undergo a further process to extract aluminum.

Europe has achieved high rates of aluminum recycling ranging from 42% of beverage cans, 85% of construction materials, and 95% of transport vehicles.[40]

Recycled aluminum is known as secondary aluminum, but maintains the same physical properties as primary aluminum. Secondary aluminum is produced in a wide range of formats and is employed in 80% of alloy injections. Another important use is extrusion.

White dross from primary aluminum production and from secondary recycling operations still contains useful quantities of aluminum that can be extracted industrially.[41] The process produces aluminum billets, together with a highly complex waste material. This waste is difficult to manage. It reacts with water, releasing a mixture of gases (including, among others, hydrogen, acetylene, and ammonia), which spontaneously ignites on contact with air;[42] contact with damp air results in the release of copious quantities of ammonia gas. Despite these difficulties, the waste is used as a filler in asphalt and concrete.[43]

The vast majority of compounds, including all Al-containing minerals and all commercially significant aluminum compounds, feature aluminum in the oxidation state 3+. The coordination number of such compounds varies, but generally Al3+ is six-coordinate or tetracoordinate. Almost all compounds of aluminum(III) are colorless.[20]

Halides

All four trihalides are well known. Unlike the structures of the three heavier trihalides, aluminum fluoride (AlF3) features six-coordinate Al. The octahedral coordination environment for AlF3 is related to the compactness of the fluoride ion, six of which can fit around the small Al3+ center. AlF3 sublimes (with cracking) at 1,291 °C (2,356 °F). With heavier halides, the coordination numbers are lower. The other trihalides are dimeric or polymeric with tetrahedral Al centers. These materials are prepared by treating aluminum metal with the halogen, although other methods exist. Acidification of the oxides or hydroxides affords hydrates. In aqueous solution, the halides often form mixtures, generally containing six-coordinate Al centers that feature both halide and aquo ligands. When aluminum and fluoride are together in aqueous solution, they readily form complex ions such as [AlF(H
2O)
5]2+
, AlF
3(H
2O)
3, and [AlF
6]3−
. In the case of chloride, polyaluminum clusters are formed such as [Al13O4(OH)24(H2O)12]7+.

Aluminum forms one stable oxide, known by its mineral name corundum. Sapphire and ruby are impure corundum contaminated with trace amounts of other metals. The two oxide-hydroxides, AlO(OH), are boehmite and diaspore. There are three trihydroxides: bayerite, gibbsite, and nordstrandite, which differ in their crystalline structure (polymorphs). Most are produced from ores by a variety of wet processes using acid and base. Heating the hydroxides leads to formation of corundum. These materials are of central importance to the production of aluminium and are themselves extremely useful.

Aluminum carbide (Al4C3) is made by heating a mixture of the elements above 1,000 °C (1,832 °F). The pale yellow crystals consist of tetrahedral aluminum centers. It reacts with water or dilute acids to give methane. The acetylide, Al2(C2)3, is made by passing acetylene over heated aluminum.

Aluminum nitride (AlN) is the only nitride known for aluminum. Unlike the oxides, it features tetrahedral Al centers. It can be made from the elements at 800 °C (1,472 °F). It is air-stable material with a usefully high thermal conductivity. Aluminum phosphide (AlP) is made similarly; it hydrolyses to give phosphine:

A variety of compounds of empirical formula AlR3 and AlR1.5Cl1.5 exist.[44] These species usually feature tetrahedral Al centers formed by dimerization with some R or Cl bridging between both Al atoms, e.g. "trimethylaluminum" has the formula Al2(CH3)6 (see figure). With large organic groups, triorganoaluminum compounds exist as three-coordinate monomers, such as triisobutylaluminium. Such compounds[which?] are widely used in industrial chemistry, despite the fact that they are often highly pyrophoric. Few analogues exist between organoaluminum and organoboron compounds other than[clarification needed] large organic groups.

Several useful derivatives of LiAlH4 are known, e.g. sodium bis(2-methoxyethoxy)dihydridoaluminate. The simplest hydride, aluminum hydride or alane, remains a laboratory curiosity. It is a polymer with the formula (AlH3)n, in contrast to the corresponding boron hydride that is a dimer with the formula (BH3)2.

AlF, AlCl and AlBr exist in the gaseous phase when the trihalide is heated with aluminum. The composition AlI is unstable at room temperature, converting to triiodide:[45]

3 AlI ⟶ AlI 3 + 2 Al {\displaystyle {\ce {3AlI->{AlI3}+2Al}}}

A stable derivative of aluminum monoiodide is the cyclic adduct formed with triethylamine, Al4I4(NEt3)4. Also of theoretical interest but only of fleeting existence are Al2O and Al2S. Al2O is made by heating the normal oxide, Al2O3, with silicon at 1,800 °C (3,272 °F) in a vacuum.[45] Such materials quickly disproportionate to the starting materials.

Very simple Al(II) compounds are invoked or observed in the reactions of Al metal with oxidants. For example, aluminium monoxide, AlO, has been detected in the gas phase after explosion[46] and in stellar absorption spectra.[47] More thoroughly investigated are compounds of the formula R4Al2 which contain an Al-Al bond and where R is a large organic ligand.[48]

Aluminum is the most widely used non-ferrous metal.[49] Global production of aluminum in 2005 was 31.9 million tonnes. It exceeded that of any other metal except iron (837.5 million tonnes).[50] Forecast for 2012 was 42–45 million tonnes, driven by rising Chinese output.[51]

Outer shells and cases for consumer electronics and photographic equipment.

Electrical transmission lines for power distribution ("creep" and oxidation are not issues in this application as the terminations are usually multi-sided "crimps" which enclose all sides of the conductor with a gas-tight seal).

Production of hydrogen gas by reaction with hydrochloric acid or sodium hydroxide.

In alloy with magnesium to make aircraft bodies and other transportation components.

Cooking utensils, because of its resistant to corrosion and light-weight.

Coins in such countries as France, Italy, Poland, Finland, Romania, Israel, and the former Yugoslavia struck from aluminum or an aluminum-copper alloy.[56][57]

Musical instruments. Some guitar models sport aluminum diamond plates on the surface of the instruments, usually either chrome or black. Kramer Guitars and Travis Bean are both known for having produced guitars with necks made of aluminum, which gives the instrument a very distinctive sound. Aluminum is used to make some guitar resonators and some electric guitar speakers.[58]

Aluminum is usually alloyed – it is used as pure metal only when corrosion resistance and/or workability is more important than strength or hardness. The strength of aluminum alloys is abruptly increased with small additions of scandium, zirconium, or hafnium.[59] A thin layer of aluminum can be deposited onto a flat surface by physical vapor deposition or (very infrequently) chemical vapor deposition or other chemical means[which?] to form optical coatings and mirrors.

Aluminum compounds

Because aluminum is abundant and most of its derivatives exhibit low toxicity, the compounds of aluminum enjoy wide and sometimes large-scale applications.

Aluminum oxide (Al2O3) and the associated oxy-hydroxides and trihydroxides are produced or extracted from minerals on a large scale. The great majority of this material is converted to metallic aluminum. In 2013, about 10% of the domestic shipments in the United States were used for other applications.[60] One major use is to absorb water where it is viewed as a contaminant or impurity. Alumina is used to remove water from hydrocarbons in preparation for subsequent processes that would be poisoned by moisture.

Being a very hard material (Mohs hardness 9), alumina is widely used as an abrasive; being extraordinarily chemically inert, it is useful in highly reactive environments such as high pressure sodium lamps.

Sulfates

Several sulfates of aluminum have industrial and commercial application. Aluminum sulfate (Al2(SO4)3·(H2O)18) is produced on the annual scale of several billions of kilograms. About half of the production is consumed in water treatment. The next major application is in the manufacture of paper. It is also used as a mordant, in fire extinguishers, in fireproofing, as a food additive (E number E173), and in leather tanning. Aluminum ammonium sulfate, which is also called ammonium alum, (NH4)Al(SO4)2·12H2O, is used as a mordant and in leather tanning,[61] as is aluminum potassium sulfate ([Al(K)](SO4)2)·(H2O)12. The consumption of both alums is declining.[why?]

Aluminium alloys with a wide range of properties are used in engineering structures. Alloy systems are classified by a number system (ANSI) or by names indicating their main alloying constituents (DIN and ISO).

The strength and durability of aluminum alloys vary widely, not only as a result of the components of the specific alloy, but also as a result of heat treatments and manufacturing processes. A lack of knowledge of these aspects has from time to time led to improperly designed structures and gained aluminum a bad reputation.

One important structural limitation of aluminum alloys is their fatigue strength. Unlike steels, aluminum alloys have no well-defined fatigue limit, meaning that fatigue failure eventually occurs, under even very small cyclic loadings. Engineers must assess applications and design for a fixed and finite life of the structure, rather than infinite life.

Another important property of aluminum alloys is sensitivity to heat. Workshop procedures are complicated by the fact that aluminum, unlike steel, melts without first glowing red. Manual blow torch operations require additional skill and experience. Aluminum alloys, like all structural alloys, are subject to internal stresses after heat operations such as welding and casting. The lower melting points of aluminum alloys make them more susceptible to distortions from thermally induced stress relief. Stress can be relieved and controlled during manufacturing by heat-treating the parts in an oven, followed by gradual cooling—in effect annealing the stresses.

The low melting point of aluminum alloys has not precluded use in rocketry, even in combustion chambers where gases can reach 3500 K. The Agena upper stage engine used regeneratively cooled aluminum in some parts of the nozzle, including the thermally critical throat region.

The statue of the Anteros in Piccadilly Circus, London, was made in 1893 and is one of the first statues cast in aluminum.

Ancient Greeks and Romans used aluminum salts as dyeing mordants and as astringents for dressing wounds; alum is still used as a styptic. In 1782, Guyton de Morveau suggested calling the "base" of (i.e., the metallic element in) alum alumine.[62] In 1808, Humphry Davy identified the existence of a metal base of alum, which he at first termed alumium and later aluminum (see etymology section, below).

The metal was first produced in 1825 in an impure form by Danish physicist and chemist Hans Christian Ørsted. He reacted anhydrousaluminum chloride with potassiumamalgam, yielding a lump of metal looking similar to tin.[63][64]Friedrich Wöhler was aware of these experiments and cited them, but after repeating Ørsted's experiments, he concluded that this metal was pure potassium. He conducted a similar experiment in 1827 by mixing anhydrous aluminum chloride with potassium and produced aluminum.[64] Wöhler is therefore generally credited with isolating aluminum (Latinalumen, alum). Further, Pierre Berthier discovered aluminum in bauxite ore. Henri Etienne Sainte-Claire Deville improved Wöhler's method in 1846. As described in his 1859 book, aluminum trichloride could be reduced by sodium, which was more convenient and less expensive than potassium, which Wöhler had used.[65] In the mid-1880s, aluminum metal was exceedingly difficult to produce, which made pure aluminum more valuable than gold.[66] So celebrated was the metal that bars of aluminum were exhibited at the Exposition Universelle of 1855.[67]Napoleon III of France is reputed to have held a banquet where the most honored guests were given aluminum utensils, while the others made do with gold.[68][69]

Aluminum was selected as the material to use for the 100 ounces (2.8 kg) capstone of the Washington Monument in 1884, a time when one ounce (30 grams) cost the daily wage of a common worker on the project (in 1884 about $1 for 10 hours of labor; today, a construction worker in the US working on such a project might earn $25–$35 per hour and therefore around $300 in an equivalent single 10-hour day).[70] The capstone, which was set in place on 6 December 1884 in an elaborate dedication ceremony, was the largest single piece of aluminum cast at the time.[70]

With the explosive expansion of the airplane industry during World War I (1914–1917), major governments demanded large shipments of aluminum for light, strong airframes. They often subsidized factories and the necessary electrical supply systems.[76]

Many navies have used an aluminum superstructure for their vessels; the 1975 fire aboard USS Belknap that gutted her aluminum superstructure, as well as observation of battle damage to British ships during the Falklands War, led to many navies switching to all steel superstructures.

Aluminum wire was once widely used for domestic electrical wiring in the United States, and a number of fires resulted from creep and corrosion-induced failures at junctions and terminations; additional and preventable factors in the failures have been identified.[77][78] Aluminum is still used in electrical services with specially designed wire termination hardware.

Etymology

The various names all derive from its elemental presence in alum. The word comes into English from Old French, from alumen, a Latin word meaning "bitter salt".[79]

Two variants of the name are in current use: aluminum (pronunciation: /ˌæljʊˈmɪniəm/) and aluminum (/əˈluːmɪnəm/). There is also an obsolete variant alumium. The International Union of Pure and Applied Chemistry (IUPAC) adopted aluminum as the standard international name for the element in 1990 but, three years later, recognized aluminum as an acceptable variant. The IUPAC periodic table now includes both spellings.[80] IUPAC internal publications use the two spelling with nearly equal frequency.[81]

Different endings

Most countries use the ending "-ium" for "aluminium". In the United States and Canada, the ending "-um" predominates.[20][82] The Canadian Oxford Dictionary prefers aluminum, whereas the Australian Macquarie Dictionary prefers aluminum. In 1926, the American Chemical Society officially decided to use aluminum in its publications; American dictionaries typically label the spelling aluminum as "chiefly British".[83][84] The earliest citation given in the Oxford English Dictionary for any word used as a name for this element is alumum, which British chemist and inventor Humphry Davy employed in 1808 for the metal he was trying to isolate electrolytically from the mineral alumina. The citation is from the journal Philosophical Transactions of the Royal Society of London: "Had I been so fortunate as to have obtained more certain evidences on this subject, and to have procured the metallic substances I was in search of, I should have proposed for them the names of silicium, alumium, zirconium, and glucium."[85][86]

Davy settled on aluminum by the time he published his 1812 book Chemical Philosophy: "This substance appears to contain a peculiar metal, but as yet Aluminum has not been obtained in a perfectly free state, though alloys of it with other metalline substances have been procured sufficiently distinct to indicate the probable nature of alumina."[87] But the same year, an anonymous contributor to the Quarterly Review, a British political-literary journal, in a review of Davy's book, objected to aluminum and proposed the name aluminum, "for so we shall take the liberty of writing the word, in preference to aluminum, which has a less classical sound."[88]

The -ium suffix followed the precedent set in other newly discovered elements of the time: potassium, sodium, magnesium, calcium, and strontium (all of which Davy isolated himself). Nevertheless, element names ending in -um were not unknown at the time; for example, platinum (known to Europeans since the 16th century), molybdenum (discovered in 1778), and tantalum (discovered in 1802). The -um suffix is consistent with the universal spelling alumina for the oxide (as opposed to aluminia), as lanthana is the oxide of lanthanum, and magnesia, ceria, and thoria are the oxides of magnesium, cerium, and thorium respectively.[citation needed]

The aluminum spelling is used in the Webster's Dictionary of 1828. In his advertising handbill for his new electrolytic method of producing the metal in 1892, Charles Martin Hall used the -um spelling, despite his constant use of the -ium spelling in all the patents[74] he filed between 1886 and 1903. Hall's domination of production of the metal ensured that aluminum became the standard English spelling in North America.

Despite its widespread occurrence in the Earth crust, aluminum has no known function in biology. Aluminum salts are remarkably nontoxic, aluminum sulfate having an LD50 of 6207 mg/kg (oral, mouse), which corresponds to 500 grams for an 80 kg (180 lb) person.[8] The extremely low acute toxicity notwithstanding, the health effects of aluminum are of interest in view of the widespread occurrence of the element in the environment and in commerce.

Health concerns

In very high doses, aluminum is associated with altered function of the blood–brain barrier.[90] A small percentage of people are allergic to aluminum and experience contact dermatitis, digestive disorders, vomiting or other symptoms upon contact or ingestion of products containing aluminum, such as antiperspirants and antacids. In those without allergies, aluminum is not as toxic as heavy metals, but there is evidence of some toxicity if it is consumed in amounts greater than 40 mg/day per kg of body mass.[91] The use of aluminum cookware has not been shown to lead to aluminum toxicity in general, however excessive consumption of antacids containing aluminum compounds and excessive use of aluminum-containing antiperspirants provide more significant exposure levels.[citation needed] Consumption of acidic foods or liquids with aluminum enhances aluminum absorption,[92] and maltol has been shown to increase the accumulation of aluminum in nerve and bone tissues.[93] Aluminum increases estrogen-related gene expression in human breast cancer cells cultured in the laboratory.[94] The estrogen-like effects of these salts have led to their classification as metalloestrogens.

There is little evidence that aluminum in antiperspirants causes skin irritation.[8] Nonetheless, its occurrence in antiperspirants, dyes (such as aluminum lake), and food additives has caused concern.[95] Although there is little evidence that normal exposure to aluminum presents a risk to healthy adults,[96] some studies point to risks associated with increased exposure to the metal.[95] Aluminum in food may be absorbed more than aluminum from water.[97] It is classified as a non-carcinogen by the US Department of Health and Human Services.[91]

Aluminum has controversially been implicated as a factor in Alzheimer's disease.[101] According to the Alzheimer's Society, the medical and scientific opinion is that studies have not convincingly demonstrated a causal relationship between aluminum and Alzheimer's disease.[102] Nevertheless, some studies, such as those on the PAQUID cohort,[103] cite aluminum exposure as a risk factor for Alzheimer's disease. Some brain plaques have been found to contain increased levels of the metal.[104] Research in this area has been inconclusive; aluminum accumulation may be a consequence of the disease rather than a causal agent.[105][106]

Effect on plants

Aluminum is primary among the factors that reduce plant growth on acid soils. Although it is generally harmless to plant growth in pH-neutral soils, the concentration in acid soils of toxic Al3+cations increases and disturbs root growth and function.[107][108][109][110]

Most acid soils are saturated with aluminum rather than hydrogen ions. The acidity of the soil is therefore, a result of hydrolysis of aluminum compounds.[111] The concept of "corrected lime potential"[112] is now used to define the degree of base saturation in soil testing to determine the "lime requirement".[113][114]

Wheat has developed a toleranceto aluminum, releasing of organic compounds that bind to harmful aluminum cations. Sorghum is believed to have the same tolerance mechanism. The first gene for aluminum tolerance has been identified in wheat. It was shown that sorghum's aluminum tolerance is controlled by a single gene, as for wheat.[115] This adaptation is not found in all plants.

Biodegradation

A Spanish scientific report from 2001 claimed that the fungus Geotrichum candidum consumes the aluminum in compact discs.[116][117] Other reports all refer back to the 2001 Spanish report and there is no supporting original research. Better documented, the bacterium Pseudomonas aeruginosa and the fungus Cladosporium resinae are commonly detected in aircraft fuel tanks that use kerosene-based fuels (not AV gas), and laboratory cultures can degrade aluminum.[118] However, these life forms do not directly attack or consume the aluminum; rather, the metal is corroded by microbe waste products.[119]

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